1. Field of the Invention
The present invention relates to a cell control device that controls the charging and discharging of cells in an electricity storage device, and to an electricity storage device that incorporates such a cell control device.
2. Description of the Related Art
In the prior art, a charging and discharging control system has been employed that controls a device for charging and discharging connected to the positive terminals and to the negative terminals of a cell group. Furthermore, with lithium ion batteries that have recently been implemented in practice, there is a possibility that a malfunction such as increase of the amount of heat generated or the like may take place if charging and discharging are not performed within an appropriate range of state of charge (SOC). Due to this, with an electricity storage device (a battery) that employs a number of lithium ion cells, a cell controller is utilized that performs cell balancing to equalize the SOCs of the various unit cells by measuring the voltage of each of the unit cells (lithium ion cells) making up the cell groups in this lithium ion battery, and by discharging the charge accumulated in certain ones of the unit cells in order to equalize the SOCs between all of the unit cells.
Furthermore, when variation in SOC between the unit cells is present, the capacity of the cell group for being charged and discharged becomes low, because the capacity for being charged is determined by those unit cells whose SOCs are the highest, while the capacity for being discharged is determined by those unit cells whose SOCs are the lowest. Moreover, although the SOC of a unit cell that has deteriorated and whose capacity has dropped increases due to charging faster than do the SOCs of the other unit cells, the deterioration accelerates further when charging is performed in the high SOC state. And, since the life of a cell group is determined by those cells that have deteriorated, accordingly variation in SOCs between the cells also shortens the life of the cell group.
As a cell balancing technique of this type, there is a per se known cell controller having a cell balancing circuit that adjusts the capacities of the various unit cells, including a cell voltage detection circuit that detects the voltage of each of the unit cells included in a cell group via voltage detection lines, resistors and switching elements for cell balancing, and a control unit that controls turning the switching elements on and off. (For example, reference should be made to Japanese Laid-Open Patent Publication 2005-348457.)
The cell controller described in Japanese Laid-Open Patent Publication 2005-348457 performs balancing by controlling balancing circuits of the type described above. In other words, it operates the balancing circuits so that the differences in SOC become smaller. This balancing operation is performed during charging and discharging of the entire assembled battery. During discharge of the entire assembled battery, the unit cells for which the balancing circuits operate have a higher proportion of balancing current in the discharge current than the unit cells for which the balancing circuits do not operate. Moreover, during charging of the entire assembled battery, the charge current for the unit cells for which the balancing circuits operate is reduced by amount of the discharge current, compared with those unit cells for which the balancing circuits do not operate. Accordingly, if the number of unit cells in the cell group whose SOCs have decreased is small, then the entire assembled battery is charged and discharged while performing balancing discharging on the remaining large number of unit cells whose SOC has not decreased, and thereby the SOC of the entire cell group is controlled to reach a target value.